1. Field of the Invention
The present invention relates to a compatible type optical pickup designed to record and/or reproduce information on and/or from optical recording media having different formats, and more particularly, to a compatible type optical pickup using a wedge type beam splitter.
2. Description of the Related Art
Generally, compatible type optical pickups are designed to record/reproduce information on/from optical recording media having different formats such as DVD and CD.
Referring to FIG. 1, a conventional compatible type optical pickup includes a light source module 11 that emits a beam having a predetermined wavelength and receives a beam reflected from an optical recording medium D, a second light source 21 that emits a beam having a different wavelength from that of the beam emitted by the light source module 11, a beam shape compensating prism 30 that changes the traveling path of an incident beam and compensates for the shape of the incident beam, a beam splitter 41 that changes the traveling path of an incident beam, an objective lens 43 that focuses an incident beam to form a beam spot on the optical recording medium D, and a main photodetector 49 that receives the beams reflected from the optical recording medium D and detects an information signal and error signal.
The light source module 11 integrates a first light source (not shown), a photodetector (not shown) disposed adjacent to the first light source, and a holographic optical element 12 into a single unit. A beam emitted from the first light source is directed through the holographic optical element 12 and travels toward the optical recording medium D. The beam reflected from the optical recording medium D is diffracted by the holographic optical element 12 and focused onto the photodetector.
A first coupling lens 13 that converges a divergent beam incident from the first light source and a first grating 15 that diffracts the beam into 0th, ±1st, ±2nd, etc. order beams are positioned in an optical path between the light source module 11 and the beam splitter 41. Here, most of the light emitted from the first light source and traveling toward the beam splitter 41 is transmitted through the beam splitter 41 and travels toward the optical recording medium D while the remaining light is reflected toward a first monitor photodetector 17. The optical power of the first light source can be checked with the amount of light received onto the first monitor photodetector 17, thereby making it possible to control the optical power of the first light source.
The second light source 21 is constituted by an edge emitting type semiconductor laser that emits light having a shorter wavelength than that of the first light source. According to the characteristics of a semiconductor laser, the second light source 21 emits a beam of an elliptic cross section. The beam shape-compensating prism 30 compensates for the shape of the incident elliptic beam in such a way as to form a circular spot on the optical recording medium D. By compensating for the shape of the beam in this way, information can be recorded employing the beam emitted by the second light source 21.
A second coupling lens 23 that converges the incident divergent beam and a second grating 25 that diffracts the incident beam are disposed in the optical path between the second light source 21 and the beam shape compensating prism 30.
The beam shape compensating prism 30 has an entrance plane 31 onto which the beam from the second light source 21 is incident, a reflection plane 32 from which the incident beam is reflected, and an exit/reflection plane 33 where the beam reflected from the reflection plane 32 is directed while the beam incident from the optical recording medium D is reflected toward the main photodetector 49. Thus, the exit beam that is transmitted through the exit/reflection plane 33 is reflected from the beam splitter 41 and travels toward the optical recording medium D.
A second monitor photodetector 27 is disposed opposite the entrance plane 31. The second monitor photodetector 27 receives some of the light that is emitted from the second light source 21 and reflected from the entrance plane 31 and detects the optical power of the second light source 21.
A collimating lens 45 that collimates an incident beam and a sensor lens 47 that adjusts the size of the incident beam and changes a focus position are placed on the optical path between the exit/reflection plane 33 and the main photodetector 49.
The compatible type optical pickup configured as above can record and/or reproduce information to and/or from optical recording media having different formats. However, the above optical pickup configuration has a problem in that the manufacturing cost is high due to the large number of optical elements, including the beam shape-compensating prism. Further, the complicated structure of the optical pickup increases the size as well as the number of assembling steps. Still another problem is that the aberration characteristics of an optical system become sensitive due to the use of a beam shape-compensating prism.